Restricted amounts of fossil fuels, such as oil and natural gas, have stimulated considerable efforts to find alternative energy sources and alternative energy carriers. Hydrogen is of great interest as energy carrier due to its high energy density and because, like electricity, it can be produced in several ways without any influence on the user of the hydrogen. Energy can be stored much easier in large quantities as hydrogen than electric energy.
As a chemical fuel, hydrogen is unique because the reaction product of a fuel cell or internal-combustion engine will be pure water and will not result in any local pollution. This gives a potential as to environmental benefits, since either can hydrogen be produced from renewable energy or the CO2 generated as bi-product in the hydrogen production can be deposited from centralized production facilities.
The storage of hydrogen gas is nevertheless a challenge, which may be accomplished under high pressure or as liquid hydrogen (−250° C.). This is, however, energy demanding and impractical, and therefore the attention is focused on the storage of hydrogen in solid substances which absorb hydrogen in their crystal lattice. This hydrogen is released by increasing the temperature, and the effort is concentrated on obtaining the largest possible hydrogen density in respect of weight and volume, and on obtaining satisfactory kinetics and costs.
Many so called interstitial metal hydrides have been made, in which hydrogen molecules are absorbed and distributed in cavities in the metal structure as single atoms, but such hydrides have so far not been able to store more than about 2.5% by weight of hydrogen. On the other hand, an other group of metal hydrides, so called complex metal hydrides, has been found to achieve higher gravimetric densities. These hydrides have complexes of a metal atom surrounded by hydrogen, AlH4− for example, incorporated into their crystal structure. Some of these hydrides have been known for more than 40 years and have been brought into extensive use as reduction agents, for example LiAlH4 and NaBH4. It has recently become known that by the use of titanium based additives, the kinetics of the dehydrogenation could be considerably improved, and rehydrogenation could be possible under moderate conditions for NaAlH4 and Na2LiAlH6. U.S. Pat. No. 6,106,801 to Bogdanovic and Schwickardi, used herein as reference, added Ti accelerator, i.a. by impregnating with Ti(OBu)4 (Bu=Butoxide) in a dietylether suspension.
More recently it has been attempted to improve the kinetics by mixing the accelerator, which is often titanium based, with the complex hydride by ball milling. The more intimate mixture and the reduced particle size contribute to improved kinetics. Further, Ti compounds with a small particle size have been found to improve the kinetics [M. Fichtner et al NanoTech. 14 (2003) 778, B. Bogdanovic et al Adv. Mat. 15 (2003) 1012]. The exact reason why particularly Ti, but also other transition metals and graphites, has such effect on the kinetics is not known, but it is now an established fact that most of the titanium is, independent of its mode of addition, reduced to metallic titanium and is then bonded up in a metastable Al1-xTix alloy with x<0.25 [H. W. Brinks et al. J. Alloys Compd. 376 (2004) 215]. One of the causes of the kinetic problems is that complex hydrides often involve two or more solid phases in dehydrogenated or rehydrogenated state so that diffusion of metal containing species is necessary in order that the reactions shall take place.
Novel groups of reversible complex hydrides have also emerged; both amides/imides of light cations such as lithium [P. Chen et al Nature 420 (2002) 302] and boron hydrides such as LiBH4. The temperature for dehydrogenation/rehydrogenation is, however, slightly higher and the kinetics, especially for the boron hydrides, is for the present inferior to Al-based complex hydrides. The potential expressed as % by weight is, however, higher for these two groups.
Bonds in interstitial metal hydrides are often via delocalized electron systems, and it is well known that one by amending the metal composition may tune the thermodynamic properties considerably; i.e. amend temperature/pressure conditions for hydrogen uptake/release. This is for example obvious for Ni which only forms hydrides at very high pressures, but by adding ⅙ La to LaNi5 one may, at a pressure of a few bars and at room temperature, form LaNi5H6.
Complex hydrides appear to contain more ionic bonds between the complex anion and the counter-ion. This lends less flexibility to amend compositions, as opposed to interstitial metal hydrides where metallic bonds lend considerably more scope for this.
There are also examples of hydrogen uptake in transition metal halides, resulting in interstitial metal hydrides, for example ThI2 into ThI2H0.7 and ThI2H1.7 [A. W. Struss, J. D. Corbett, Inorg. Chem. 20 (1978) 965]. This happens by a different pressure than metallic Th and demonstrates that stabilization adjustments by halogenides may be obtained also for other types of metal hydrides.
For complex hydrides there exists only one known and well characterized example of a possible substitution of the cations, namely by substituting ⅓ of Na in Na3AlH6 with Li so that Na2LiAlH6 is obtained [H. W. Brinks et al. J. Alloys Compd. 392 (2005) 27]. This increases the stability of an already far too stable compound and there are no possibilities of gradual adjustment of the Li-content of the phase. LiMg(AlH4)3 has also been characterized, but structure and stability of this compound are not known.
A theory on the mechanism of titanium for increased reversibility of NaAlH4 was a solid solution on Na or Al position [D. L. Sun et al J. Alloys Compd. 337 (2002) L8], which would result in that the thermodynamic stability was amended. However, intensive studies with a great selection of samples with synchrotron X-ray diffraction and neutron diffraction give no indication of alteration of the crystal lattice [H. W. Brinks et al. J. Alloys Compd. 376 (2004) 215]. Further, measurements of pressure-composition isotherms show, under approximate equilibrium conditions for Na2LiAlH6 at a temperature so high that Ti accelerator is not necessary, concurrent equilibrium pressures [J. Graetz et al. Phys. Rev. B 71 (2005) 184115] for samples with and without addition of Ti.
A possibility to circumvent this problem has recently been suggested by adding thermodynamic favourable side reactions which contribute to lower the stability of the total reaction [J. J. Vajo et al. J. Phys. Chem. B 109 (2005) 3719]. The reaction LiBH4═B+LiH+ 3/2H2 was for example destabilized by adding MgH2 so that MgB was formed: LiBH4+MgH2=MgB+LiH+ 5/2H2. This does not effect the stability of the LiBH4 phase itself, but the side reaction MgH2+B═MgB+H2 changes the thermodynamic characteristics of the total reaction. A disadvantage of this method is that many phases are involved with two condensed phases on either side of the reaction equation. Moreover, the temperature must go up to about 300° C. where LiBH4 is in molten state and where catalyzed MgH2 merely by its high % by weight has appropriate equilibrium pressure and excellent kinetics.
But even though NaAlH4 theoretically gives 5.6% by weight hydrogen in a two-step reaction, LiNH2+2LiH gives 10.2% by weight hydrogen and LiBH4 gives 13.9% by weight hydrogen, the kinetics of both dehydrogenation and rehydrogenation, and the temperature by which the reaction takes place, are still not satisfactory for hydrogen storage systems for inter alia vehicles. It is therefore substantial room for improvement of hydrogen storage in solid materials.